Resin transfer molding (RTM) is a process wherein dry fiber reinforcement plys/preforms are loaded in a mold cavity defining the configuration of the article to be fabricated and saturated/wetted by a flowable, thermoset resin. Saturation is effected by introducing the resin into the mold plenum under a pressure differential that causes the resin to flow through and saturate the fiber reinforcement preforms. Representative examples of RTM processes are described in U.S. Pat. Nos. 4,942,013 and 4,891,176.
The primary advantage of the RTM process resides in its potential for high rates of production. Other advantages include the capability to accurately locate internal details, to control outer and inner mold line surfaces and part thicknesses, and savings in material and labor. Use of the RTM process, however, has typically been limited to low strength, simple configuration articles (as compared to high strength, complex configuration aerospace structures) due to difficulties associated with the RTM process and RTM molding assemblies.
Conventional RTM molding assemblies consist of matched metal male/female molds (tools) made from rigid machined metal. Matched metal molds are expensive to fabricate, especially where such molds include internal mandrels that are necessary to form complex configurations such as reverse flanges. Molds for such complex configurations must be fabricated in intricate geometric patterns to facilitate mandrel insertion/removal, and such elaborate geometric configurations require increased labor and time expenditures to fabricate the basic mold assembly.
Other difficulties associated with the RTM process and apparatus include stabilizing and debulking the fiber reinforcement preforms for use in the RTM process, and loading the preforms into the mold cavity. It is often necessary to "build-up" the mold assembly around the fiber reinforcement preforms, a labor intensive and time consuming procedure. Maintaining adequate tolerances may be difficult. Matched metal molds have locked-in cross sections that make mold closure and sealing difficult due to the bulk of the fiber reinforcement preforms.
In addition, large complex fiber reinforcement preforms subjected to resin transfer molding often experience partial resin curing prior to saturation of the entire preform, resulting in partially-saturated composite structures of diminished strength. To compensate for this effect, prior art RTM processes have utilized large pressure differentials to ensure saturation of the entire preform.
Large pressure differentials, however, may result in composite structures having fiber-to-resin ratios on the order of about 30 to about 45 percent. This is in contrast to composite structures formed from prepregs which have fiber-to-resin ratios of about 60 to about 65 percent, such ratios providing composite articles having good structural strength. A low fiber-to-resin ratio, in contrast, results in a composite structure having reduced structural strength. For RTM processes, it is accepted practice to apply a 20% knockdown factor on the mechanical properties of the finished composite article in acknowledgement of the lower fiber-to-resin ratio.
With regard to fabricating complex configuration composite articles, the aforedescribed difficulties may result in disorientated fibers, areas which are resin rich or lean, depending upon bulk variations of the preform, and/or porosity. Any of these conditions reduces the specific strength of the composite article. Due to process and materials costs, rejection of a single fabricated article may negate the cost savings available by utilizing the resin transfer molding method.